1. Field of the Invention
The present invention relates to imaging apparatuses, and exposure control apparatuses, methods, and programs. More specifically, the present invention relates to an imaging apparatus for capturing an image using a solid-state imaging device, and an exposure control apparatus, method, and program for controlling an exposure adjustment mechanism when capturing the image.
2. Description of the Related Art
In imaging apparatuses using solid-state imaging devices, such as digital still cameras and digital video cameras, generally, luminance information is detected for various types of photographing operation control such as exposure control. An imaging apparatus of the related art is disclosed in Japanese Unexamined Patent Application Publication No. 02-56180 in which a luminance level is detected for each of a plurality of areas from a captured image signal to perform exposure adjustment based on the luminance level of a priority area, and a gamma correction value is changed according to the contrast of each area to reduce overexposure and underexposure problems in a non-priority area.
In the imaging apparatus of the related art, a captured image signal is subjected to various types of image-quality correction based on digital computation such as white-balance adjustment. Recently, linear-matrix processing to enhance the color reproduction has attracted attention.
In the imaging apparatus of the related art, luminance information is detected at one specific location in a signal processing path, such as a location subsequent to a linear-matrix computation unit or a location subsequent to a white-balance adjustment unit, and photographing operation control and the like are performed using the detected luminance information. However, exposure control based on luminance information detected at one location encounters a problem in that it may be difficult to provide accurate exposure control, as described below.
FIG. 5 shows the relationship between the amount of light and the chrominance and luminance levels in an output signal from an imaging device.
Generally, exposure control is realized by determining a target luminance control value according to luminance information detected from a captured image signal and an exposure-control value based on various exposure settings and performing feedback control on exposure adjustment functions, such as an aperture diaphragm and an automatic gain control (AGC) device, so that the target luminance control value and the luminance information are matched. If the luminance information has a larger value than the target luminance control value, the aperture diaphragm is closed to reduce the amount of light directed to a light-receiving unit, thereby preventing overexposure. If the luminance information has a smaller value than the target luminance control value, the aperture diaphragm is opened or the gain is increased to increase the amount of light directed to the light-receiving unit, thereby preventing underexposure.
Such exposure control is based on the assumption that the luminance information has a large value when the amount of light directed to the light-receiving unit increases. In a typical light-receiving apparatus that converts incident light from an object into RGB signals, however, the relationship between the amount of incident light and the output signal of the light-receiving unit is obtained as shown in FIG. 5. That is, once light of more than a predetermined amount is incident, the output signal saturates. Further, the amount of light that saturates the output of each RGB color depends on the transmittance of the color filter or the sensitivity characteristic of the light-receiving device.
The luminance information for use in exposure control is generally represented by a weighted mean value of the signal values for RGB. As shown in FIG. 5, as the RGB output signal levels (chrominance levels) of the light-receiving unit saturate, the luminance signal level (luminance level) also saturates. If an amount of light sufficient to saturate the output signals of the light-receiving unit is incident, the outputs of the light-receiving unit do not change even when the exposure adjustment function is controlled, resulting in no change in the luminance information. Thus, accurate exposure adjustment is not realized.
In order to avoid the above-mentioned problem, the following control operations are performed. If the luminance level exceeds a predetermined value or does not change, it is determined that the amount of light is outside the controllable range, and the above-described feedback control is terminated until the amount of light falls within the controllable range. If the luminance level is considerably small, it is determined that the amount of light is below the controllable range, and the above-described feedback control is terminated until the amount of light falls within the controllable range.
Actually, an image is displayed on the screen after performing signal processing, such as linear-matrix computation or white-balance adjustment, on an output image signal of the light-receiving unit. If chrominance information is adjusted by such signal processing, a luminance signal generated from the chrominance information also has a different value from the luminance signal output from the light-receiving unit. In particular, a large change in the signal caused by linear-matrix computation or white-balance adjustment may result in incorrect exposure on a displayed (or recorded) image even through the exposure for the output image of the light-receiving unit is accurate. Therefore, a system in which luminance information is detected at only a location prior to the signal processing unit has a problem in that accurate exposure control may not be provided.
FIG. 6 shows the relationship between the amount of light and the chrominance and luminance levels in an image signal subjected to linear-matrix computation using a negative parameter.
In the linear-matrix computation, a computation parameter (matrix coefficient) is often set to a negative value to enhance the color reproduction. In a case where the effect of the negative parameter is large, the RGB signal levels and luminance level subjected to the linear-matrix computation vary in the manner shown in FIG. 6, and there appears a phenomenon in which, within a certain amount of light, the luminance level decreases with an increase in the amount of light. In this case, if exposure control similar to that described above is performed using luminance information detected from an image signal subjected to linear-matrix computation, for a region in which the luminance level decreases with an increase in the amount of light, the amount of light directed to the light-receiving unit is increased although the amount of light directed to the light-receiving unit should be restricted. As a result, accurate exposure control is not realized. Further, it fails to determine whether or not the amount of light is outside the controllable range.
When exposure control is performed using luminance information detected from an image signal subjected to white-balance adjustment, the luminance information may be changed due to the change in white-balance gain even though the exposure is correct in an environment where the amount of light directed to the light-receiving unit does not change, thereby suppressing accurate exposure control. Therefore, exposure control may not be accurately carried out even when luminance information is detected from an image signal subjected to linear-matrix computation or white-balance adjustment.